Laser communication in space

In outer space, the communication range of free-space optical communication[1] is currently of the order of several thousand kilometers,[2] suitable for inter-satellite service. It has the potential to bridge interplanetary distances of millions of kilometers, using optical telescopes as beam expanders.[3]

Demonstrations and tests

1991-2000

In 1992, the Galileo probe proved successful one-way detection of laser light from Earth as two ground-based lasers were seen from 6 million km by the out-bound probe.[4]

2001-2010

In November 2001, the world's first laser data connection was achieved in space by the European Space Agency satellite Artemis, providing an optical data transmission link with the CNES Earth observation satellite SPOT 4.[5]

In May 2005, a two-way distance record for communication was set by the Mercury laser altimeter instrument aboard the MESSENGER spacecraft. This diode-pumped infrared neodymium laser, designed as a laser altimeter for a Mercury orbit mission, was able to communicate across a distance of 24 million km (15 million miles), as the craft neared Earth on a fly-by.[6]

In 2008, the ESA used laser communication technology designed to transmit 1.8 Gbit/s across 45,000km, the distance of a LEO-GEO link. Such a terminal was successfully tested during an in-orbit verification using the German radar satellite TerraSAR-X and the American NFIRE satellite. The two Laser Communication Terminals (LCT)[7] used during these tests were built by the German company Tesat-Spacecom[8] in cooperation with the German Aerospace Center (DLR).[9]

2011-today

In January 2013, NASA used lasers to beam an image of the Mona Lisa to the Lunar Reconnaissance Orbiter roughly 390,000km (240,000mi) away. To compensate for atmospheric interference, an error correction code algorithm similar to that used in CDs was implemented.[10]

In November 2013, laser communication from a jet platform Tornado was successfully demonstrated for the first time. A laser terminal of the German company Mynaric (formerly ViaLight Communications) was used to transmit data at a rate of 1 Gbit/s over a distance of 60km and at a flight speed of 800km/h. Additional challenges in this scenario were the fast flight maneuvers, strong vibrations, and the effects of atmospheric turbulence. The demonstration was financed by EADS Cassidian Germany and performed in cooperation with the German Aerospace Center DLR.[13][14][15]

In November 2014, the first ever use of gigabit laser-based communication as part of the European Data Relay System (EDRS) was carried out.[16] Further system and operational service demonstrations were carried out in 2014. Data from the EU Sentinel-1A satellite in LEO was transmitted via an optical link to the ESA-Inmarsat Alphasat in GEO and then relayed to a ground station using a conventional Ka band downlink. The new system can offer speeds up to 7.2 Gbit/s.[17]

In December 2014, NASA's OPALS announced a breakthrough in space-to-ground laser communication, uploading at a speed of 400 megabits per second. The system is also able to re-acquire tracking after the signal is lost due to cloud cover.[18] The OPALS experiment was launched on 18 April 2014 to the ISS to further test the potential for using a laser to transmit data to Earth from space.[19]

In February 2016, Google X announced to have achieved a stable laser communication connection between two stratospheric balloons over a distance of 62 miles (100km) as part of Project Loon. The connection was stable over many hours and during day and nighttime and reached a data rate of 155 Mbit/s.[20]

In June 2018, Facebook's Connectivity Lab (related to Facebook Aquila) was reported to have achieved a bidirectional 10 Gbit/s air-to-ground connection in collaboration with Mynaric. The tests were carried out from a conventional Cessna aircraft in 9km distance to the optical ground station. While the test scenario had worse platform vibrations, atmospheric turbulence and angular velocity profiles than a stratospheric target platform the uplink worked flawlessly and achieved 100% throughput at all times. The downlink throughput occasionally dropped to about 96% due to a non-ideal software parameter which was said to be easily fixed.[21]

Future missions

Laser communications in deep space will be tested on the Psyche mission to the main-belt asteroid 16 Psyche, planned to launch in 2022.[22] The system is called Deep Space Optical Communications,[23] and is expected to increase spacecraft communications performance and efficiency by 10 to 100 times over conventional means.[23][24]

Operators

Multiple companies want to use laser communication in space for satellite constellations in low Earth orbit to provide global high-speed Internet access. Similar concepts are pursued for networks of aircraft and stratospheric platforms.

Suppliers

A substantial market for laser communication equipment may establish when these projects will be fully realized.[36] New advancements by equipment suppliers is enabling laser communications while reducing the cost. Beam modulation is being refined, as its software, and gimbals. Cooling problems have been addressed and photon detection technology is improving. Currently active notable companies in the market include:

Secure communications

Secure communications have been proposed using a laser N-slit interferometer where the laser signal takes the form of an interferometric pattern. Any attempt to intercept the signal causes the collapse of the interferometric pattern.[41][42] This technique has been demonstrated to work over propagation distances of practical interest[43] and, in principle, it could be applied over large distances in space.[41] Assuming available laser technology, and considering the divergence of the interferometric signals, the range for satellite-to-satellite communications has been estimated to be approximately 2,000km.[44] These estimates are applicable to an array of satellites orbiting the Earth. For space vehicles or space stations, the range of communications is estimated to increase up to 10,000km.[44] This approach to secure space-to-space communications was selected by Laser Focus World as one of the top photonics developments of 2015.[45]